Experiments are proceeding in an attempt to identify different species of motoneurons in the pool supplying a single muscle. We have recently succeeded in demonstrating the existence of two distinct species of motoneurons among those that make up the portion of the pool that contributes to critical firing levels from 0 to 8 percent. These two types of cells can be distinguished by their maximal rates of firing. Other properties of these cells are being examined in order to discover other ways in which they may differ. An attempt is also being made to determine whether the size-principle applies more precisely within each single species or within the entire pool. BIBLIOGRAPHIC REFERENCES: Clamann, H.P., Gillies, J.D., Skinner, R.D., and Henneman, E.: Quantitative Measures of Output of a Motoneuron Pool during Monosynaptic Reflexes. J. Neurophysiol. 37: 1328-1337, 1974. Henneman, E., Clamann, H.P., Gillies, J.D., and Skinner, R.D.: Rank Order of Motoneurons within a Pool: Law of Combination. J. Neurophysiol. 37: 1338-1349, 1974.